Interferometric optical sensing is a subject of interest in a variety of fields for many different applications. In particular, optical vibration sensors are used to detect ultrasonic signals in different non-contact monitoring of vibrating surfaces, non-destructive testing of engineering products, for non-invasive monitoring of medical parameters, for a variety of scientific investigation of delicate or inaccessible materials and devices, and for other purposes. Optical detection offers many advantages, in particular, non-contact, wide-bandwidth and high sensitivity. Optical detection methods, and especially interferometers, however, suffer from high sensitivity to environmental effects, such as instability and vibrations in the test arrangement, and temperature variations. The environmental instabilities lead to drift of the interferometer from its optimal operation point, which lead to loss of signal intensity, and signal distortion. Such have been overcome with heterodyne and super-heterodyne demodulation methods. These solutions require optical frequency modulation and specialized optics, making them, as other state of the art solutions, relatively bulky and expensive.
Several methods to address the problem of interferometer instability at its “work-point”have been suggested previously. U.S. Pat. No. 4,109,818 (Vilkomerson) introduced two reference beams in phase quadrature in a single interferometric setup. The two beams are generated either by inserting a phase-shifting plate to cover a portion of the reference beam, or through phase-shifting one linear polarization in a 45° polarized beam. The former method is impractical due to alignment difficulties: the substantial difference in the optical paths of the reference beam portions; and the introduction of vibration noise through the difficulty to stabilize the relative location of the phase-shifting plate and the reference beam. The latter method is complex and also suffers from the need for high precision alignment as well as stability requirements.
U.S. Pat. No. 7,298,497 (Millerd et al.) stipulates an interferometric imaging system for sensing optical wavefronts split into four interferometric images detected with a single camera. To this end the image and the reference wavefronts are angularly separated by a diffractive component (holographic optical element) into multiple images, the reference of each shifted to a different phase multiple of 90°. Millerd et al. (as well as several workers referenced therein) mathematically combine the four phase-shifted interferograms to solve explicitly for the phase difference between the object and reference wavefronts.
The present invention proposes use of simple, low-cost readily manufacturable and compact components and offers both high sensitivity and good immunity from environmental effects.